Above-ground storage tanks are those which are supported on the ground rather than buried or supported in elevated position. Such tanks can be of substantial size and volume and may range to a football field in diameter or more.
New and rebuilt ground storage tanks use an environmental safety secondary containment liner in the form of a plastic membrane. The liner or membrane is usually spaced a short distance beneath the metal bottom which is supported on compacted earth above and below the liner. The membrane is designed to contain leaks to prevent ground contamination.
Unfortunately, because of the dielectric properties of the secondary containment liner, conventional and widely accepted cathodic protection methods such as those using deep anodes or distributed anodes is no longer applicable. Such systems usually use large anodes which, in any event, would not normally fit in the relatively narrow envelope between the liner and the tank bottom. The dielectric liner effectively blocks the required current flow from such anodes to the tank bottom. Accordingly, to be effective, an anode system has to be placed in the relatively narrow envelope between the liner and the tank bottom.
Many operators have chosen galvanic cathodic protection systems which use zinc or magnesium ribbon anodes. These galvanic anodes ribbon systems are typically installed in parallel lengths between the membrane and the tank bottom floor. This method of cathodic protection can be an effective means of tank bottom corrosion control. However, because of the large volume of anode material required to protect fully the tank bottom, these systems have proven to be quite costly, or are in a reduced configuration which is not capable of providing effective corrosion protection. In addition, the life of galvanic anode systems is limited and usually not commensurate with the design life of the tank.
Also, because most ground storage tanks use complex and highly sensitive leak detectors, it is important that the compacted medium between the liner and tank bottom not be comprised of hydrocarbons or not be carbonaceous. Experiments have shown that conventional electronic conductive carbonaceous backfills, which are widely used with existing impressed current systems, set off such leak detectors or otherwise render them useless. The backfill must not be an electronic conductor to avoid shorting between the anode and bottom, and yet must be capable of being compacted and supporting uniformly high compressive loads. The backfill material, however, must be an electrolytic or ionic conductor.
It is also important that the anode be generally uniformly spaced from the tank bottom and not touch the bottom. If it touches, a short occurs and the system malfunctions, or if it is not substantially uniformly spaced from the tank bottom a near short occurs resulting in non-uniform distribution of the protective current. The area beneath a large ground storage tank is hardly accessible, and convenient repairs are virtually impossible. It is, therefore, important to use as anode materials, components which don't themselves substantially corrode, or which don't form current blocking oxidation layers. Further, the anode and the connections to the anode should provide a thin or low profile and should also be such that the system provides a minimal cathodic protection current substantially uniformly to the entire tank bottom.